Which of these numbers is prime? ${5,\ 40,\ 45,\ 63,\ 81}$
Answer: A prime number has exactly two factors: 1 and itself. The factors of 5 are 1 and 5. The factors of 40 are 1, 2, 4, 5, 8, 10, 20, and 40. The factors of 45 are 1, 3, 5, 9, 15, and 45. The factors of 63 are 1, 3, 7, 9, 21, and 63. The factors of 81 are 1, 3, 9, 27, and 81. Thus, 5 is a prime number.